The project aims at providing an insight into understanding the structural and molecular basis of remarkable diversity of catalytic activity and expression observed in cytochrome P450s. First of all, we established a bacterial expression system of mouse P450 so as to provide enough purified P450s for various analyses. NMR: 19F-Trp-labeled P450s were purified in an attempt to understand P450-steroid interactions. Crystallization: An attempt has been continuing to monomerize the bacterially-expressed and purified P450s using various detergents. Site- directed mutagenesis: Using various steroids as substrates, we identified three key amino acid residues that determine the regio- and stereospecificities. These residues also play a key role in the metabolic activation of procarcinogens. Molecular Modeling: Modeling steroids in the substrate pocket of bacterial P450 cam is providing a unifying picture of how the key residues may determine specificities. We identified a regulatory element containing a CpG site in the 5' -flanking region of the sex-specific P450 genes and found that CpG site in the male-specific genes is preferentially demethylated in adult males, while a site in the female-specific gene exhibits female-preferential demethylation. We have characterized the heteromeric transcription factor GABP as a potential activator only when the corresponding CpG site is demethylated. A nuclear factor which binds only to the methylated element is also present and is under investigation. Phylogenetical comparison of the 5"-flanking sequences revealed a single nucleotide substitution which abolishes binding of a nuclear factor to a regulatory element of the non sex-specific, orthogenic P450 gene in the wild mice. We purified this nuclear factor (NF2d9, the mouse homologue of human LBP- 1a which modulates HIV-1 transcription) and cloned its cDNA. NF2d9 may be a negative transcription factor and its binding to a regulatory element appears to be regulated by a JAK 2-dependent phosphorylation.